texts: Calculating the Size of Earth in the Apollo 8 Earthrise Photo

texts:
I was debating with someone online (I know, great way to get nowhere fast) regarding the size of Earth as it appears in the famous 'Earthrise' photos from Apollo 8.
Below is an accurate (almost pixel-perfect) calculation of the apparent size Earth should appear. In this instance it checks out, however if there is a better approach in calculating this and similar images I would be interested to know :

The photograph was shot on 70mm film using a 250mm lens. We can calculate the what the Field of View should be for the image, using the following formula :

         FOV (rectilinear) = 2 * arctan (frame size/(focal length * 2))

i.e.
             FOV (rectilinear) = 2 * arctan (70/(250 * 2))
             = a FOV of 15.93 degrees.

From our point of view on Earth, the average angular diameter of the moon is 0.5 degrees. From the Moon, the Earth has an angular diameter of about 1.9 .

From this it can be calculated how many pixels in size the Earth should appear in the photograph :

Diameter of Earth in Pixels = Photo's Diagonal Resolution * (Earth's Angular Diameter / FOV)

i.e.

Diameter of Earth in Pixels = 3841.87 * (1.9 / 15.93)

= 458.23 pixels

For reference, The full, un-cropped version of the 'Earthrise' photo that the above calculation is basing the image resolution upon can be found at this location :

https://www.nasa.gov/images/content/297755main_GPN-2001-000009_full.jpg

I imagine that this could be further refined by deriving the correct angular diameter of Earth from the exact distance of the moon at the time the photo was taken, as well as the altitude of the Apollo module above the moon.


texts: Distance from the sun can equal a hospitable planet?

texts:
It is said that planets are continuously moving away from the sun. At one time Mars was just the right distance to support life at least temperature wise?
Could Venus be far enough from the sun eventually to cool down?

Related:
Are planets moving away from the sun?

Is Earth losing orbit?


texts: Clarification of radio spectrum terminology

texts:
For a radio spectrum, when is it called thermal emission, synchrotron emission, self-absorbed synchrotron emission and inverted spectrum? They are all power-law and their difference is power-law index?
For a power-law $\nu^{\alpha}$, as long as $\alpha$ is positive, the spectrum is an inverted spectrum, right?
Please recommend reference books which introduce these models and their corresponding astrophysical phenomena.


texts: Could Pluto and Charon have extra-Solar origin?

texts:
Pluto and Charon seem to have surprisingly young surfaces, considering that resurfacing events were expected to be rare for them.

Could the explanation be that they actually are young? Formed later than the Solar system at some other star and then ejected and captured? Or that they are ancient but have extra-Solar origin and have little cratering because they have spent billions of years as a vagabond planet system in empty interstellar space? Sedna is believed to be captured.


texts: Calculate angle of inclination for a certain angle in a planet's orbit?

texts:
The background is that I'm trying to write a computer program to show at what angles each planet would be able to be seen transiting in front of the sun from an observer outside our solar system.  My endgame is to calculate the relative percentages of the sky that could see any group of planets transiting together.  For instance in the direction the Earth is from the sun in June and December, an alien observer that could see Earth transit could also see Venus transit, but from the angle where the Earth is in September or March they would not ever be able to see Venus transit.

I wrote a program using basic stats and circular orbits to test out the idea, but I would like to make it more robust using actual elliptical orbits and I'm having trouble.  For circular orbits with the sun at the center I was able to just take the inclination of Earth's orbit times the sin of the viewing longitude relative to the longitude of ascention.

double viewingLongitude = longitudeRadians - LongitudeOfAscent;
double planetAngle = Inclination * Math.Sin(viewingLongitude);

For instance Earth's orbit has an inclination of 1.57 degrees and longitude of ascent at 348.74 degrees.  At 348.74 degrees and 168.74 degrees Earth will have an actual inclination of 0 degrees.  At 78.74 degrees and 158.74 degrees Earth will have an actual inclination of 1.57 and -1.57 degrees from the plane.

Is there a similar easy way I could calculate that inclination for the elliptical orbit of the Earth or will I have to solve Kepler's equations?


texts: Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?

texts:
I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored in vibrational and rotational modes after each collision. They state that the gas was so hot that regular atomic $H$ generated enought pressure to impede gravitational collapse and that, only because of $H_2$, it was possible to lower that pressure and stars could form.
My question is, if $H_2$ were impossible to form (for some reason) wouldn't gravitational collapse eventually happen as the universe expanded and the gas cooled down? Or the expansion itself would be enought to prevent this by lowering the density of the gas before any star could be formed?
Would a universe without $H_2$ had formed any stars in its entire evolution?


texts: What would happen if a Sun-like star were to consume a Jupiter-like planet?

texts:
A substantial number of discovered exoplanet systems are likely contain more than one gas giant with distances far closer to the star than in our solar system.

It is known that in solar system evolution gravitational effects between objects can lead to some of them migrating outwards while others migrate inwards.

Has there been any study of what happens when a gas giant approaches its star close enough to start losing mass to it? In this case could the planet end up completely in the star?

For the purposes of this question let's choose a solar-mass G-type star and a gas giant such as Jupiter, since we have so much first-hand knowledge of these objects.

Jupiter has plenty of hydrogen and this would represent the addition of more nucleosynthetic fuel.

Would this be enough to cause the Sun to become the red giant in this case?


texts: In ecliptic coordinates, what are the ranges of ecliptic longitude and latitude in radians? Do they both go from $-\pi$ to $\pi$?


texts: Are Symbiotic and Vampire Stars the same type of star or different?

texts:
Are Vampire Stars and Symbiotic Stars the same type of star or is there a difference?

Vampire Stars siphon off hydrogen from its neighbour.  According to https://www.cfa.harvard.edu/~kenyon/ss/, Symbiotic Stars seem like the same description.

"In all systems, the hot component - a main sequence star, a white dwarf, or a neutron star - accretes material lost by the red giant." (Harvard)

SS Leporis has been described as Symbiotic Star and in some places as Vampire Stars.


texts: Which structure is represented in this video (if any)?

texts:
I know this is not supposed to be precise since it is just a movie but I would suppose it is based on real things. In the Contact movie intro
https://youtu.be/EWwhQB3TKXA?t=1m I think the Milk Way is shown at around 2:12, I am wondering what is the structure shown around 2:32. Does that a representation of anything real? Virgo maybe?


texts: Why do pre-main sequence stars show lithium in absorption?

texts:
This paper and this paper state without discussion that the presence of lithium absorption in the stars they observe as strong evidence of these stars being pre-main sequence stars. Because they state this without any discussion and justification, I assume that it's a commonly known and accepted fact.  The second of the two papers also state without discussion or justification that some observed strong H$\alpha$ emission was also evidence of the stars being pre-main sequence stars.

Googling around I was able to get a partial answer for why this is so.  Pre-main sequence stars retain their primordial lithium as they collapse, but at some point around when they become main sequence stars, the lithium in their photospheres is destroyed by fusing with protons. I haven't been able to find anything describing why H$\alpha$ emission is associated with pre-main sequence stars, other than this has been known for a very long time (since before at least 1945).

I am wondering

What is it about becoming a main sequence star that causes the lithium to fuse with protons near the surface?
How fast is the lithium destroyed, and does this destruction occur right when the star enters the main sequence?  I'm wondering how definite the presence of lithium absorption is in saying whether a star has entered the main sequence or not.
Why is H$\alpha$ emission such a prominent feature of pre-main sequence stars?


texts: What should be the "poles" for irregular shaped bodies?

texts:
Continuation of: What is the definition of a "pole" of a celestial body?

From uhoh's answer, we can conclude that a distinct bodies should have a center of mass. If the body is spherical, then the COM will be near the middle of the body. By definition, the body's rotational axis should pass though COM and the extreme ends of this rotational axis should be considered "poles".
But what about bodies whose COM lies outside the body itself? For instance, COM for crazy C-shaped objects lies outside the body, the axis of rotation does not intersect the body's surface and so technically, there are no "poles" for such bodies. So, how should we describe the "extreme ends" of the body? How are COM measured for such bodies? Is there any list for such bodies where COM lies outside the bodies?
Note: There are a few bodies where there are no stable poles but they have their reasons. Saturn's moon Hyperion and the asteroid 4179 Toutatis lack a stable north pole. They rotate chaotically because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether.


texts: Is the vernal equinox always in zenith somewhere on the equator?

texts: I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is always on the equator. And I suppose it usually is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?


texts:
As I was curious about the exact identical comment to my title here: https://astronomy.stackexchange.com/a/16135/10102

And as it seems no one asked it yet, I'm simply interested in the difference of the both, so:

What's the distinction between "planetary formation mechanisms" and "gravitational collapse"?


texts: What is this bright "glow" in the center of galaxies?

texts:
It was always my belief that at the center of many galaxies, there are supermassive black holes. If this is the case, then we should not see a "light" coming out from the center since light get's sucked in black holes. Doing a quick search of galaxies on google image, I came across this:

These are famous galaxies, and real, and so I was wondering what this very white-yellow glow is in the center. If it is a collection of stars, then why are there so many stars in the center of a galaxy? If there is a black hole there, then why is there light?


texts: Why has Venus's atmosphere not been stripped away by solar wind?

texts:
This answer on Space Exploration to a question about Mars says that one reason Mars has such a thin atmosphere is because it lacks a magnetic field to protect it from the effect of double solar winds.
Here MBR explains that Venus does not have a magnetic field.

Image credit: ESA
If this is the case, then why has Venus's atmosphere not been stripped away by solar wind like that of Mars?


texts: Effect of black coating on the inside of a hobby telescope's tube?

texts:
I was watching this (rather old) episode of The Making, where they show how a telescope is built in a factory. At 7:34, the cylindrical body of the telescope is shown to have a dark coating on the inside and the video explains that this prevents internal reflection of the light received from whatever you're observing.
I saw an interesting comment from Izumikawa Fukumi who asked if the telescope would become more accurate if they coated the inside with the blackest black in the world. How would such a dark coating affect the telescope's performance?


texts:
This answer to Farthest distance to a solar system object that's been measured by radar? mentions that Saturn's rings, and the Uncover Travel post Arecibo Observatory, Puerto Rico – The World’s Largest Radio Telescope For Over 50 Years mentions:

Among other accomplishments of the Arecibo Observatory are:

Direct imaging of an asteroid for the first time in history.
Discovery of water ice deposits at Mercury’s poles.
Tracking of near-Earth asteroids to monitor impact risks.
Mapping the cloud-covered surface of Venus.
Radar imaging of the rings of Saturn, revealing new details of the ring structure.
First detection of methane lakes on Titan, a moon of Saturn.
First detection of an asteroid with a moon.

Question: How did Arecibo detect methane lakes on Titan, and image Saturn's rings? These are pretty remarkable feats from Earth for a single radiotelescope. How were they done? Can citations be found and the examples of the image of the rings and methane lake evidence be shown?


texts: Formula to Compute sigma8 for correction in non-linear regime

texts:
I try to explain my issue on this forum since it seems to be appropriate.

I need to apply a correction on $\sigma_{8}$ between linear and non-linear regime to keep it fixed (I make change the values of cosmological parameters at each iteration). I have to compute $\sigma_{8}$ from the $P_{k}$ (generated by CAMB code) and found the following relation (I put also the text for clarify  the  context) :

  Part of this Klein Onderzoek is aimed at finding an estimate of the
  cosmological parameter $\sigma_{8}$ from peculiar verlocity data only.
  $\sigma_{8}$ is defined as the r.m.s. density variation when smoothed
  with a tophat-filter of radius of $8 \mathrm{h}^{-1} \mathrm{Mpc}
> .[9]$ The definition of $\sigma_{8}$ in formula-form is given by:

  $$ \sigma_{8}^{2}=\frac{1}{2 \pi^{2}} \int W_{s}^{2} k^{2} P(k) d k $$

  where $W_{s}$ is tophat filter function in Fourier space:

  $$ W_{s}=\frac{3 j_{1}\left(k R_{8}\right)}{k R_{8}} $$

  where $j_{1}$ is the first-order spherical Bessel function. The
  parameter $\sigma_{8}$ is mainly sensitive to the power spectrum in a
  certain range of $k$ -values. For large $k,$ the filter function will
  become negligible and the integral will go to zero. For small $k,$ the
  factor $k^{2}$ in combination with the power spectrum factor $k^{-3}$
  will make sure that the integral is negligible. In other words,
  $\sigma_{8}$ is mostly determined by the power spectrum within the
  approximate range $0.1 \leq k \leq 2 .$ since $\sigma_{8}$ is only
  sensitive to a certain range of $k,$ any difference in the values of
  the Hubble uncertaintenty, the baryonic matter density and the total
  matter density will influence the found estimate.

Question 1) What numerical value have I got to take for $R_{8}$ in my code : for the instant, I put $R_{8}= 8.0/0.67$ : is this correct ?

Question 2) The other issue is, for each correction on $A_{s}$, that I find with this expression a value roughly around : $\sigma_{8} = 0.8411 ........$ instead of standard (fiducial) value  $\sigma_{8} = 0.8155 ........$ : there is  a 4 percent of difference between both values : is the expression above right ?

Could anyone tell me a good way to compute $\sigma_{8}$ from $P_{k}$ generated by CAMB-1.0.12 ?

Regards


texts: We are not able to quantize spacetime and the theory of  loop quantum gravity has been proved wrong but we still use the Planck length in astrophysics. Why?


texts:
How far is the Earth/Sun above/below the galactic plane, and is it heading toward/away from it?
We know the Sun is orbiting the galactic center every 225 million years.
But how far are we from the galactic plane, and is the inclination of the plane of the Sun's orbit around the galactic center, as compared to the galactic plane, known?

According to this drawing

the Sun is traveling approximately in the ecliptic plane.
However, the ecliptic plane is steeply inclined to the galactic plane, so if this drawing is correct (from voyager.jpl.nasa.gov) then this would mean the plane of the Sun's orbit around the galactic center is also quite inclined.
And thus my question, what is the distance to the galactic plane, are we heading toward/away from it, and what is the inclination of the orbital plane of our Sun/solar system to the galactic plane? 3 questions I guess.


texts: What’s the object between the Earth and The Sun currently showing in Google maps?

texts:
If I select satellite imagery in Google maps and zoom out, the Earth is shown in space. It shows light/dark regions correctly updated, but there’s an object between the Earth and the sun. It looks too small to be The Moon.

Is it real? What is it?


texts: What would night sky look like if Earth was made of antimatter

texts: Let's assume that Earth in its current state is suddenly replaced with the Earth completely identical, but made entirely of antimatter. Ignoring meteorites and asteroids which would cause mass destruction, extinction events and whatnot, what would the night sky look like as the anti-Earth swept up the space dust?


texts: Why aren't secondary mirrors offset to get rid of diffraction spikes due to the support vanes?

texts:
Some kind of ellipsoidal shape mirror could reflect to a secondary mirror which is not in the way of the infalling light. Two advantages are immediately obvious. The primary mirror would not be partly shadowed by the secondary. And the support structure of the secondary would not cause diffraction spikes in point sources.

What are the reasons for why this is not only not the standard, but quite uncommon? Especially on the greatest telescopes, one would think that the additional production difficulties, if any, are not a relevant consideration given the budget and clever engineers behind them.


texts:
What is the canonical method to calculate the surface brightness limit of a 2D image, e.g., in a 1 arcsec2 aperture?
...if image has erg/s/cm2 units and each pixel has some arcsec2 dimension.


texts: Can red dwarf stars have a spectral type of L?

texts: We have the "normal" main sequence stars, OBAFGKM. Below K and M are the brown dwarfs, L,T, and Y. Zooming into the area at the edges of M and L, we have spectral types M9, M9.5, L0, and L1. So I am wondering, is there any red dwarf that has a spectral type of L?


texts: Sum of the Masses of All Moons in the Solar System

texts:
I'm looking for the total mass of the moons in the solar system in Earth Mass
this would be include the 67's moons of jupiter, the 62's of saturn, the 27's Uranus and the 14's for Neptune.

it's a crazy question I know, if you have some list of the object with mass in same range i can make the addition ^^


texts: Why do the planets in our solar system orbit in the same plane?

texts:
(Yes I'm excluding Pluto from this the same way it was excluded for not being a planet)

Observing the planets orbit of the Sun they all seem relatively planar and roughly all orbit along the same plane.

Is this due to the way that our Solar system was formed or is this a physical phenomena observed in other systems?


texts:
I understand that a neutron star is dense enough that adding more matter will increase the amount of degenerate matter directly, and the limit to its size is about 1.4 solar masses.

But if an object were formed not by an explosion that crushes it down to neutron matter, is it necessarily so?

Given normal atoms of any desired species, and it is piled up carefully so it does not get hot. When it gets around one solar mass, will it necessarily collapse into a neutron star, or can it get more massive?  Will such an object eventually collapse catastrophically or can it have a degenerate matter core and a substantial thickness of less-compressed matter on top and normal matter on top of that?

What is the largest possible mass of a cold solid object?  Can it be several solar masses?

By cold I mean that it’s not a star, puffed up by ongoing consumption of energy. Whatever it's made out of is not being “used up”.


texts: Do astronomers measure radio waves absorption to know the shape of a celestial body?

texts: I do not have much knowledge about radio astronomy. I know that astronomers know an objects' shape by measuring the reflected photons that reach radiotelescopes, but can they measure also the absorbed ones for the same purpose?


texts: Discrepancy between my results of Great Conjunction 2020 and Wikipedia

texts:
Using python-skyfield to calculate the upcoming conjunction of Jupiter and Saturn. I'm off a few minutes with the times on Wikipedia.
Wikipedia Great conjunction times (1800 to 2100)
Using Right Ascension:

Dec 21,2020 13:22:00 UTC - Wikipedia.
Dec 21,2020 13:34:33 UTC - My Calculation.

Using Ecliptic Longitude:

Dec 21,2020 18:37:31 UTC - Wikipedia
Dec 21,2020 18:20:40 UTC - My Calculation.

from skyfield.api import load, tau, pi
from skyfield.almanac import find_discrete

planets = load('de421.bsp')
sun = planets['sun']
earth = planets['earth']
jupiter = planets['jupiter barycenter']
saturn = planets['saturn barycenter']

ts = load.timescale(builtin=True)

def longitude_difference(t):
    e = earth.at(t)
    j = e.observe(jupiter).apparent()
    s = e.observe(saturn).apparent()
    _, lon1, _ = s.ecliptic_latlon()
    _, lon2, _ = j.ecliptic_latlon()
    return (lon1.degrees - lon2.degrees) > 0

def longitude_difference1(t):
    e = earth.at(t)
    j = e.observe(jupiter).apparent()
    s = e.observe(saturn).apparent()

    jRa, _, _ = j.radec()
    sRa, _, _ = s.radec()
    return (sRa._degrees - jRa._degrees) > 0

longitude_difference.rough_period = 300.0
longitude_difference1.rough_period = 300.0

print()
print("Great conjunction in ecliptic longitude:")
t, b = find_discrete(ts.utc(2020), ts.utc(2021), longitude_difference)
for ti in t:
    print(t.utc_jpl())

print()
print("Great conjunction in right ascension:")
t, b = find_discrete(ts.utc(2020), ts.utc(2021), longitude_difference1)
for ti in t:
    print(t.utc_jpl())

Can anyone verify the correct UTC time?  Is there a bug in my code?


texts: How did Eratosthenes know that the sun is far away?

texts:
The famous measurements and calculations done by Eratosthenes around 300 BC are very widely known. He concluded correctly that the circumference of the Earth is about $252\,000$ times the length of an athletic stadium.
But what Eratosthenes did would make no sense if the Sun were (for example) only $6000$ miles from the Earth. How did he know it was much farther away than that?


texts: How long does it take a star to transition from main sequence to red giant?

texts: I have read that it takes billions of years for stars of approximately the size of the Sun to get through the main sequence, after which it will enter the red giant phase. When that happens, how long does it take for the star to balloon outward when it becomes a red giant?


texts: What are the challenges for the building and data analysis of the CHIME telescope?

texts:
The  Canadian Hydrogen Intensity Mapping Experiment (CHIME) has discovered the second repeated fast radio burst recently. However, its structure is simple apparently. I wonder why we did not build a telescope before. What is the challenge? I just know that its data volume is large.
It is difficult to update the telescope?
There are 4 cylinders and 1024 antennas. What determines the distribution of the 1024 antennas? I mean why is it not just one longer cylinder?


texts: How do I see more than just points in the sky?

texts:
I just bought a new telescope with these specifications:

Aperture: 203 mm
Focal Length: 1200 mm
F/ratio: f/5.9

I usually use a 28 mm eyepiece, giving me about 42.5x magnification, but I also have a 20 mm and a 10 mm eyepiece.

I was trying to find Messier objects, but I only found the Orion Nebula, and even then it wasn't very impressive. I was wondering, what types of objects are see-able in my telescope? I read that Charles Messier found the Messier objects using a four-inch refractor, and I have an 8 inch reflecting telescope.

When I tried to find things like the pinwheel galaxy, or the crab nebula, I only saw points of light. So basically, how do I see things like nebula and galaxies?
For reference, I live in the midwestern United States.


texts:
I have two series of velocities (HJD, $v_{1}$ and $v_{2}$ ), how to fit in detail?

There is any formula on the web? $sine$ curve is easy, but I need to consider an eccentric orbit.

Suppose we can estimate $m_{1}$, $m_{2}$, and inclination angle $\approx80^{\circ}$, if we just know velocities of both of the two companions, which parameters can me derive?

What is the difference between binary velocity fitting and star+planet fitting?


texts: What is the temperature of an accretion disc surrounding a supermassive black hole? Is there plasma in the disc?


texts: Is there any ephemeris generator that gives you accurate distances between Earth and other bodies?

texts:
I've tried JPL's HORIZONS ephemeris generator and while it gives accurate celestial coordinates for an object it doesn't give the geocentric distance.
JPL's Solar System Simulator gives the distance to an object at a certain time however only down to a few digits.
Is there an accurate tool that generates exact (meaning down to the meter) geocentric distances between Earth and another Solar System body over increments of time?


texts: Would a red dwarf star resemble our own Sun at sunset to an observer on a nearby planet?

texts:
Suppose you're observing a red dwarf star at Noon from the surface of an Earth-like planet orbiting in the red dwarf's habitable zone, so that the red dwarf's luminosity is equal to our own Sun.
Aside from the red dwarf's greater perceived size in the sky, would its color and brightness closely resemble our own Sun at sunset in Earth's sky on a clear, dust and smoke-free evening?
Is it fair to say that since the Sun's color and brightness changes as it dips closer to the horizon due to scattering of blue light, there could conceivably be a close color/brightness match to any G, K, or M class star at some point along the Sun's trajectory in the sky?


texts:
In my research on exoplanets, I have heard many people talk "forward modeling of exoplanet atmospheres".  I don't know what the "forward" means in "forward modeling" and how it compares with "reverse modeling", if that's even a thing.

What is forward modeling, and why is it so special that it needs to be distinguished from just plain ol' regular modeling?


texts: Are meteors and meteorites considered "Small Solar System Bodies"?

texts:
The difference between meteorites, meteors, and meteoroids is one of altitude relative to a celestial surface: in space, it's a meteoroid; in the atmosphere, it's a meteor; and on the surface, it's a meteorite. (See here and here for more information.)
The International Astronomical Union (IAU) defined the term Small Solar System Body (SSSB) in 2006 with Resolution B5 as

(3) All other objects³,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
[Footnote 3] "These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs),comets, and other small bodies."

And, although this official definition clearly states, "All other objects ... orbiting the Sun", I'm not sure how verbatim this definition is meant to be treated.
What I mean by this is, well, for example, in Law, all definitions are, by default, unless previously stated, treated in an context of exactness. But although science is oftentimes based on specificity, it is not always, and the degree thereof in this context is thus not wholly clear to me.


texts: Does interstellar cloud obstruct or reduce the visibility?

texts:
We just pictured a medium distant black hole by combining data from various observatories around the world. That's when I wondered if interstellar cloud blocks the visibility at multiple position in the same direction in space such that we never accurately know the exact luminosity and distance between the black hole and Earth? I was not able to find much information about this particular subject so,

Is it possible that the interstellar cloud reduces the visibility of an object behind it?
And if so, is it possible that we are only able to observe interstellar cloud upto a certain distance in space?
If we observe a star that is behind a interstellar cloud, how do we determine the luminosity of the star?
Can we accurately filter out interstellar cloud that are separated by x distance but at the same direction?


texts: If the density of nebular distribution increases symmetrically in all directions, how does space curvature explain it?

texts:
Edwin Hubble, in The Observational Approach to Cosmology, wrote

  Thus the density of the nebular distribution increases outwards,
  symmetrically in all directions, leaving the observer in a unique position. Such a favoured position, of course, is intolerable.

My first question is: do more recent observations still show a symmetrical nebulae density increase? Or did Hubble just do a miscalculation, misinterpretation of an observation etc.?

Apparently, Hubble can solve this problem with spatial curvature. Further below, he writes

  Therefore, in order to restore homogeneity, and to escape the horror of a unique position, the departures from uniformity, which are introduced by the recession factors, must be compensated by the second term representing effects of spatial curvature. - There seems to be no other escape.

I understand how an expanding Universe explains the symmetrical redshift, but I fail to understand how it explains the symmetrical increase of nebulae density.


texts: I saw a blinking light source, can someone explain it?

texts:
I was out watching the Perseids in the AM hours of August 11, 2018 from central Hungary. I brought my binoculars just in case I want to do regular stargazing. There was a blinking light directly overhead around Zeta Cephei. At first, I thought it was a tumbling satellite or debris, but:

It did not move anywhere. Even when observing with binoculars, it stayed in one spot, blinking like an airplane.
It was just 1 color (white).
The surrounding stars were not flashing.
This one wasn't particularly bright. The magnitude seemed randomized (with each blink).
There was a regular-ish pattern of 1-2 second intervals, best way to describe it is "digital", like a beacon, not a gradually fading brightness.

Things I'd rule out:

I'm sure this wasn't Twinkling (turbulent atmosphere), I know what that looks like.
If it were a balloon or a drone, it would have moved slightly.
If it were a geosynchronous satellite, it would have been on the celestial equator to the south (from my location), but they are so far away I doubt I could see them.

Eventually the blinking lost brightness and the object either disappeared or blended in with the non-blinking stars. I lost track where and which it was, alternating naked eye vs. binoculars. What do you think it was?


texts: Do Satelites travel in train like formation?

texts: This morning approx 5.30am i seen roughly 12 travelling in perfect formation approx 15cm apart , im  200km south of sydney Australia travelling from a north west direction


texts: Can we make reliable models to estimate what the angular momentum was in preplanetary stellar disks?

texts: I have recently become aware of how important the angular momentum of a preplanetary disc may have been in determining what type of a planetary system a star may have. We are able to measure the spin rate of many stars (to what error factor I'm not sure). I'm wondering if, through modelling, we can estimate what disc angular momentum may have been based on remaining angular momentum in the star. Have we looked at systems where we have found multiple planets and also have a good idea of the spin rate of the star to see if there are any correlations?


texts: How do we know an event is a gamma-ray burst?

texts:
How do we know an event is a gamma-ray burst (grb)?

There are two kinds of grbs, long burst and shot burst, which are caused by supernova explosions explosion and the merger of two neutron stars, respectively. However, at least one grb, J1644, is a tidal disruption event. Is there any other grb which was not caused by a supernova or the merger of two neutron stars?

When can we say that a supernova explosion is a grb after observations? How do we know an event is a grb especially when we missed its explosion?

Can we tell that the afterglow is caused by a grb?

Please give me some reference papers since I am a layman of this field.


texts: If fusion only happens near the center of the sun, what's with all the "fire" on the surface?

texts:
I understand that it isn't "fire" in the combustion of oxygen sense, but I can't find an answer online about what we're witnessing happening at the visible surface of the sun if all actual fusion explosive stuff is occurring near the center.

Would love a primer or a link to an explanation.


texts: How to calculate the current distance between two planets

texts:
I'd like to add a feature to one of my apps that calculates the current distance between the Earth and Mars (or any other planet, for that matter). My google searches have led me to statistics of average distances (as a planets orbit is elliptical, and therefore the distance can vary greatly), as well as articles about Keplers laws of Planetary motion and more. I have also seen some mentions of HORIZONS, but looking over it I did not see anything to fulfill my questions.

I haven't found any readings explicitly describing how to calculate the current distance between two planets. So, how is this done?

Please explain your answer and the steps needed, so that I may understand better. Pseudocode is welcome.

Thanks!


texts: Moving objects in the sky tonight 27th March 2020

texts:
Multiple moving objects not in a line. The are staggered single line and in groups of two or three. One or two moved downward. They had a dim light orangey colour.

It was not obvious that these satellites as normally observed.

Also observed witnessed whilst on the mobile phone with family people in the street.

Moving from the area of canis minor travelling left two or three minutes apart. There were over 50 of them.

What could have it been?


texts:
This evening I have been thinking about the anxiety caused in 1982 by predictions in John Gribbin and Stephen Plagemann 1974 book The Jupiter Effect.
I was a teenager in 1982. The planets aligning somewhat together on one side of the Sun did not cause any problems on Earth.
I am curious to know how frequently the same 5 planetary positions recur, say Mercury in Aries, Venus in Scorpio etc., when we restrict ourselves to the 5 naked eye planets.  (By the way, I do not believe in astrology; I am just trying to find the date of an ancient text that quotes planetary positions).
Say, if the orbital periods of Mercury, Venus, Mars, Jupiter and Saturn are 1/4, 2/3, 2, 12 and 30 years, then the least common multiple (LCM) is 60 years.  So, the same planetary positions will repeat every 60 years.  Is my logic correct, though my orbital periods are not exact?  I know very well that the 5 visible planets do not have the resonant orbital periods that I have mentioned above, so I am sure that the same planetary positions do not recur every 60 years.  If I used the correct orbital periods, will I get the correct answer to my question, whatever the correct answer may be?  The above Wikipedia article says that the 1982-type planetary alignment occured in 1128 AD. One of the article's references says that all planets more or less line up behind each other on the same sector of the sky once every 500 years.
Thus, my real question is: Is there a simple method to calculate (without using astronomy software) how frequently the same combination of planetary positions recur?
The method does not have to be too accurate.


texts: Is **Voyager I’s** reduced data transmission rate as described in [this article][1] because of the distance or because its transmitter getting slower?

texts: Is Voyager I’s reduced data transmission rate as described in this article because of the distance or because its transmitter getting older & slower?


texts: Compute Planet's Apparent Visual Magnitude

texts:
In a hypothetical solar system there exist:

a sun of radius $r_s$ and absolute visual magnitude $V$
the Earth, with radius $r_e$ and distance from the sun of $1\,AU$
another planet, wit radius $r_p$, albedo $a$, distance from Earth $d_e$ (in units of $AU$), and distance from the sun $d_s$ ($AU$)

Assume that the second planet is magically lit up entirely by the sun (so it does not have phases when viewed from Earth).

Now how can I compute the apparent visual magnitude of the second planet with respect to Earth?

If there are other parameters required, please tell me.


texts: Is this the best non-radio image of whatever's at the center of M87? How was it taken?

texts:
The BBC News article First ever black hole image released shows the instantly-iconic radio image of the (almost certainly) supermassive black hole at the center of the M87 galaxy, just released by the Event Horizon Telescope collaboration, also shown below.

Though it calls it a "first image", the article also shows a similarly edible-looking† donut-shaped image taken earlier, presumably using a different technology. However the caption doesn't explain what that is.

†https://i.stack.imgur.com/J5TEe.gif

Does anyone recognize the earlier image and/or know what imaging technology and wavelengths were used to construct it? Is it roughly the same scale? Are these comparable donut-hole images?

  Astronomers have suspected that the M87 galaxy has a supermassive black hole at its heart from false colour images such as this one. The dark centre is not a black hole but indicates that stars are densely packed and fast moving. Credit: DR JEAN LORRE/SCIENCE PHOTO LIBRARY

  The first ever picture of a black hole: It's surrounded by a halo of bright gas. Credit: EHT

Overly entheusiastic Wikipedia edit?


texts:
From what I understand, the TRAPPIST 1 system was discovered using the transiting method looking at 20 ultra-cool small stars. I'm not sure how many have been observed since this study.

What I wonder is, what would we expect to see if all 20 had planets? I'm not sure how many planetary systems in this and future similar studies we would expect to see edge on. What about radial velocity measurements?

Did we just get lucky with TRAPPIST 1?


texts: Could a contracting Universe create the redshift effect observed by Hubble?

texts:
What if we just can’t see far enough and the redshift actually shows the galaxies speeding up towards a gigantic black hole to where everything is converging, repeating a cycle that took place countless times “before” the big bang?

Complete text in this link: http://www.therealebook.com/Astronomy/Redshift_ContractingUniverse.html

Even in a contracting Universe, there is a region around the observer where all the galaxies are moving away; the galaxies that are moving towards the observer could be too far to be seen. If we can demonstrate this, we could challenge the conclusion that the Universe is expanding, based on the redshift effect observed in the “nearby” galaxies.

Let’s start with a simpler representation: 1 dimension only (1D). Assume that all the galaxies move along a single line. We could represent an expanding and a contracting Universe as per the images below:

In Situation 1, we have the currently accepted model, where galaxies move away from each other, at increasing speed, which creates the redshift observed by Hubble.

In Situation 2, we assume that in the contracting Universe the galaxies have a speed that is inversely proportional to the distance from the center (as in a sink flow where the particles accelerate towards the drain), galaxies 1 and 3 will display a red shift effect, because they are moving away from each other, while galaxies 4, 5 and 6 will appear with blueshift effect. However, in order to see the blue shifted galaxies it is necessary to see beyond the center of the Universe, which would be not possible either because of the distance or because of the gigantic black hole in the center traps light from galaxies 4, 5 and 6 preventing it from reaching us at galaxy 2.

In fact, assuming that from galaxy 2 we cannot see beyond galaxy 1, it is impossible to distinguish between situation 1 and 2.
With this we have demonstrated that for a 1D Universe it is impossible to determine if it is expanding or contracting, simple because we cannot see far enough. What about a 2D space?

Consider two galaxies (1 and 2); consider that they form a angle beta, relative to the center of the universe (assuming that there is one); Consider that galaxy 1 is at the distance r from the center; consider that galaxy 2 is at a fraction of r (say xr) from the center.

We can calculate the distance between galaxy 1 and 2 as follows:

From the equation above we can calculate alfa and from  we can calculate d.

We also suppose that the speed along the radius can be calculated with the equation v = c / r, where c is constant.

Therefore we can calculate the position of the two galaxies after a period of time t, and measure the distance d between them to verify if d increases (redshift) of decreases (blueshift).

We can see that no matter what the fraction x is, there is always a minimum value for the angle beta, such that any galaxy within this angle will be moving away from the reference galaxy. Now assume that the universe is so big that we cannot see galaxies beyond beta. In this way, all that we can see is redshifted galaxies!

This means that even for a contracting universe, depending on how far we can see, it is possible to detect only redshifted galaxies. Does not this disprove the theory of expanding universe based on redshift?


texts:
From my understanding, it is believed that almost every big galaxy and especially spiral galaxies have supermassive black holes (SMBH's) at the center. Also, from what I've read, a SMBH isn't required for a galaxy to exist since and in layman's terms, the matter inside a galaxy such it gas clouds and formation of stars will keep it gravitational-ly in check.

If I am right or even wrong, then would there be significant difference if our galaxy did not have a SMBH at the center? Significant enough for noticeable differences even here on Earth?


texts: Formula for sun's atmospheric refraction below the horizon

texts:
I have searched in Google and found interesting articles for atmospheric refraction influence on sun's elevation above the horizon (from 0° till 90°).
eg. Bennett G.G. etc.
I would like to know if there is a formula that approximates the refraction when the sun's center is below the horizon (from 0° to ca. -3°) and taking in account for temperature and air pressure.


texts: Is it possible to measure galactic red shift using consumer telescope equipment?

texts:
I was wondering if it would be possible to measure red shift from distant galaxies using consumer grade telescope equipment and spectroscopy filters. (like this one)

I imagine it would require a tracking mount, and enough aperture to gather data on distant faint objects, but I wonder if it would require more sensitive equipment then a consumer grade tools available for under 5-6 thousand.

A long term goal would be to replicate Hubble's work, and directly measure the expansion rate of the universe from backyard equipment.


texts:
I'm looking for a simple algorithm to compare astronomical images (of the same sky region) against each other, compute their movement and rotation, to finally stack them.

At the moment I'm already having a more or less working algorithm. First I extract all the stars out of an image (including information like brightness and FWHM), and then I walk through all the resulting "points" and create triangles out of the current point and those two other stars that have the shortest distance to this star.

This list of triangles is created for every image. After this I take one image as a reference and then I walk through the list of triangles in the reference image to find a triangle in the other image with the same length of each side of the triangle (I also "allow" some tolerance due minimal relative differences of the star positions in each image). For this matches I calculate movement and rotation relative to the reference image. The last step is to find the matched triangles that have to same relative movement and rotation like the other matches. This is done by calculating the standard deviation, sorting out triangles that are not within 1 or 2 sigma and repeat this process until I have a very small standard deviation.

The last part, finding "valid" triangles with the same movement/rotation, is working fine. The problem is that sometimes I have only like 2 or 3 "valid" triangles out of 300 initial triangles. All other triangles have side lengths different to those of the reference image.

So I assume it's the way I generate my initial triangles which causes the problem. Sortings stars by their brightness and using this data to generate the triangles also doesn't work. So is there a better way to create the initial triangles in all the images?


texts: Can Kepler's 3rd Law be applied in higher dimensions to describe orbits (quadric surfaces)?

texts:
Motivation for asking:

The 2-D orbital path of a celestial body revolving around another celestial body due to the force of gravity can be described by a conic section; this is the curve formed by intersecting a plane with the surface of a cone. This generalized conic section is given by:

$A x^2 + B xy + C y^2 + D x + E y + F = 0$

The ellipse is one example of a conic section (others being parabola and hyperbola); this happens when $B^2 - 4 A C < 0$.

Kepler's 3rd Law can be applied to elliptical orbits; this relates the orbital period $T$ and the semi-major axis $a$ of the orbit such that:

$\frac{a^3}{T^2}$ $=$ $\frac{G (M + m)}{4 \pi^2}$

As I understand it, Kepler's Law holds when the orbital path is 2-D. What happens when the orbital path is 3-D? I am aware that the conic section in 3-D is referred to as a quadric surface, which is given by:

$A x^2 + B y^2 + C z^2 + D xy + E xz + F yz + Gx + Hy + Jz + K = 0$

Say one were to rotate the 2-D orbit by its polar angle in spherical coordinates - that is, the altitude (as opposed to azimuth) - by a non-zero amount such that the orbit was in 3-D (changing z-coordinates as a function of time). Doing this, we can generate an ellipsoid, which is given by: $(\frac{x}{a})^2$ + $(\frac{y}{b})^2$ + $(\frac{z}{c})^2$ = 1.

Question:

Could the logic behind Kepler's 3rd Law be used to relate the orbital period $T$ of a body on an ellipsoidal path to any of the ellipsoids parameters $a, b, c$? What if the path weren't ellipsoidal, but some other quadric? Can the quadric surface always be reduced to a rotate 2-D conic section? Or are ALL orbits considered Keplerian?


texts: How to Interpret Lomb Scargle periodogram

texts:
lomb scargle periodogram:

import numpy as np
import pandas as pd
from matplotlib import pyplot as plt
from scipy import signal
from astropy.time import Time
from astropy.timeseries import LombScargle

m5_data = pd.read_csv('tmp')
m5_data.head()

     date    sales
0  2011-01-29  32631.0
1  2011-01-30  31749.0
2  2011-01-31  23783.0
3  2011-02-01  25412.0
4  2011-02-02  19146.0

"""Converting dates to MJD """
tmp_str = m5_data.iloc[:,0].astype(pd.StringDtype())
t_date = np.array(tmp_str.values,dtype = 'str')
t_date = Time(t_date, format='isot', scale='utc')
t_date.format = 'mjd'

y = m5_data.iloc[:,1]
m5_ls = LombScargle(t_date, y)
m5_frequency, m5_power = m5_ls.autopower()
plt.plot(m5_frequency,m5_power)
plt.show()

What does the x axis represent? Is it frequencies in days or is it frequencies in 1/day?
So the spike at 2 does that mean there is a period of every two days or does it mean there is a period 1/2 days, which is a fractional day and I do not understand how I could get fractional days when I only have 1 observation per day.

The overall goal is to find if the data is periodic. Given this data is sales of a product from a store I would expect that if the data is periodic that the periods would be 7days(weekly), 30days(monthly), 182days(semi annually)
So the spike at 2 does not make sense to me.


texts: Affect of leap seconds on the prediction of an eclipse

texts:
There seem to be irregularities in the motion of the earth, so we add "leap seconds" to adjust our time accordingly. Since 1972 there have been 26 leap seconds I think. If we extrapolate that backwards 0.5 leap seconds per year, it would be 1 leap minute in 120 years and 5 leap minutes in 600 years.

Would this be enough to throw off a prediction of an eclipse? In other words, could someone in 1416 predict an eclipse right now given the unpredictable errors caused by leap seconds?


texts: Relation between angular resolution and aperture?

texts:
The angular resolution of a telescope is given by $\frac{1.22\lambda}{D}$.

This should mean that the smaller the aperture D, the higher the angular resolution. But it is exactly the opposite - why?


texts: Could the Moon naturally capture a large asteroid and keep it as a subsatellite? For how long?

texts:
(Edited at James K's recommendation to stay on-topic for this SE.)

From my reading on this SE and from Googling I've gotten conflicting reports on the viability of subsatellites this close to the Sun, but a naive application of the Hill Sphere equation suggests that if it got captured at all, a large asteroid could theoretically orbit the moon in a stable orbit.

What I don't know is this:

For how long? Longer than a couple hundred thousand years, such that humans on Earth wouldn't have known anything else? Longer than a million years? Probably not longer than a billion, due to the tidal effects decaying the asteroid's orbit, but is that a bad assumption?
How eccentric would the asteroid's orbit be? Would it be unlikely for it to orbit with negligible eccentricity, whether on the ecliptic or not? If so, what kind of eccentricity/orbital inclination would be reasonable?


texts:
I heard that Saturn has 62 moons with confirmed orbits, 53 of which have names and only 13 of which have diameters larger than 50 kilometers. Some come within 1km of each other.

How do these moons keep in Saturn's orbit without hitting each other, and someday will these moons collide with each other and be destroyed?


texts: Why do planets revolve around the sun? I understand that gravity plays a huge role in allowing it to move around. But what caused it to revolve in the first place? One possible solution is explained with the theory of relativity but is there a classical mechanics approach to this question?


texts: If the big bang is true, after the emission of light from the hydrogen plasma, the universe was still expanding. Why would we expect to see uniform radiation if earth very well could have formed outside of this hydrogen plasma. Then we would expect to see bits of background radiation coming from one direction. People sometimes answer this question by saying that the big bang happened everywhere and that where the earth is right now was a haze of hydrogen plasma 13.7 billion years ago. I do not see how one can assume this as since the universe expands we could have formed outside of the plasma and therefore should not expect to detect uniform background radiation.


texts: Which binary star system has a greatest angular distance?

texts: Visible from the Earth, which binary star system has greatest angular distance ?


texts: Does photon capture in a black hole (or any other "very compact" object) reduce the luminosity/photon flux from its self isotropic emission?

texts:
I don't know if this is even a feasible question or if I have a point in asking so.
Let's say we can model the radiation coming from a very compact object, such as a BH with an accretion disc or any star with proper dimensions (for example, with its disc/gravitational radius within its photon sphere $R_\gamma = 1.5 R_s=3M$), as a black body radiating isotropically. I guess we can do so, since we cannot imagine any good reason for discarding this approximation in them!
What I try to ask is basically if we should modify the formula for blackbody emission or whatsoever bolometric magnitude/luminosity in such an object, given that we already know a radius and an effective temperature (we are using $L = 4\pi \sigma R^2T^4$ here). And also considering that due to the strong curvature some photons emitted in the surface of the disc/atmosphere will be returning inside and will never reach us...
Due to bending and lensing, supposedly we have only a sector from the compact object that will be sending photons outside, cf. MTW pp. 675 or Shapiro & Teukolsky pp. 352:
when measuring the subtended emission angles from a local static observer.
What I'm sure to know and explain with my own words is that the emitted photon flux reduces when we measure it as receivers on Earth, due to this capture.


texts: Evidence for homogeneity of the universe

texts: Cosmologists assume that the universe is homogeneous on large scales and, in particular, that the unobservable parts are similar to the observable universe. As I understand it, one important reason for this assumption is simply something along the lines of “hey, it would be really strange and surprising if our corner of the universe turned out to be special.” While this does seem very reasonable to me, I was wondering what other reasons (if any) we have for the assumption. Is it, for instance, predicted by the big bang model?


texts:
Question: How will microshutter arrays be used in the James Webb and future space telescopes? Are they acting as a sort of moving pinhole or slit, or is the pattern more complicated, like a coded aperture telescope? I can't imagine that would work though, as the application seems to include spectroscopy rather than just image reconstruction.

The NASA Goddard news item NASA to Demonstrate New Star-Watching Technology with Thousands of Tiny Shutters says:

The technology, called the Next-Generation Microshutter Array (NGMSA), will fly for the first time on the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy, or FORTIS, mission on October 27. The array includes 8,125 tiny shutters, each about the width of a human hair, that open and close as needed to focus on specific celestial objects.

and

The microshutter technology gives scientists the ability to produce multiple spectra at once. This capability improves productivity on both sounding rocket missions, which offer only six minutes of observing time, or large space-based observatories, which can take up to a week to observe faint, far-away objects and gather enough light to obtain good spectra. With observing time at a premium, the ability to gather light from multiple objects at once is paramount.
Webb, scheduled to launch in 2021, will carry NASA's first-generation microshutter technology — four 365-by-172 microshutter arrays that together total 250,000 shutters. They will allow Webb to obtain spectra of hundreds of objects simultaneously.
What distinguishes the next-generation array on FORTIS from the one flying on Webb is how the shutters are opened and closed. Webb’s arrays employ a large magnet that sweeps over the shutters to activate them. However, as with all mechanical parts, the magnet takes up space and adds weight. Furthermore, magnetically activated arrays can’t be easily scaled up in size. As a result, this older technology is at a disadvantage for supporting future space telescopes larger than Webb.
Magnet Eliminated
To accommodate future missions, Goddard’s microshutter-development team eliminated the magnet. The shutters in the pilot 128-by-64 array that will fly on FORTIS open and close through electrostatic interactions. By applying an alternating-current voltage to electrodes placed on the frontside of the microshutters, the shutters swing open. To latch the desired shutters, a direct current voltage is applied to electrodes on the backside.
Without a magnet, the next-generation array can be dramatically scaled up in size — and that’s precisely what the team is attempting to accomplish. Particularly, Greenhouse and Li are using advanced manufacturing techniques to create a much larger, 840-by-420 array equipped with 352,800 microshutters, dramatically increasing an instrument’s field of view.
“The array that is flying on FORTIS is a technology development prototype for the big one,” Greenhouse said.


texts: Would a telescope that uses Fresnel lenses be more practical than using regular lenses?

texts:
If we were to make telescopes with Fresnel lenses instead of regular lenses, would it be more practical?  Would this also allow for much bigger telescopes to be made?

Here is an example of the type of lens I am talking about from https://commons.wikimedia.org/wiki/File:Fresnel_lens-1.svg


texts: At what depth on Mars would the atmosphere have equal pressure of that on Earth?

texts: I know the atmospheric pressure on Mars is less than that of Earth, and so is its gravity.  However, I know that the deeper you go (e.g. in a cave or a hole that is dug), the more atmospheric pressure you would experience.  How deep would you have to go on Mars (e.g. below the surface) for the atmospheric pressure to be equal to the pressure we experience on the surface of the earth?


texts: magnetic shield for spacecraft outside of Earths shield

texts: I'd like to know if it's possible to create a strong enough onboard magnetic field, similar to Earths' that would protect deep space spacecrafts from the solar wind?


texts: How much does being at the perihelion heat Earth up?

texts:
The author in this article: https://news.nationalgeographic.com/news/2010/07/100707-aphelion-heat-wave-earth-sun-science-weather-nation/ has explained that positional differences on the orbit is a negligible factor - about 7% more sunlight at the perihelion compared to the aphelion. I suppose the explanation is relatable to people in the Northern/Southern hemispheres, where temperatures are driven by changing seasons.

However, for people in the equator, where daily temperature swings measure only in a matter of few degrees all year round, it would have definitely made some difference. Does anyone know if this temperature change has been investigated before? I would like to read up more about it. Thanks!


texts: What would the moon look like to someone at the South Pole?  To a first approximation, it's bisected by the horizon. But how far above and below does it get?


texts: Is there a computer simulation that anyone knows of that describes the loss of a moon as it's orbit extends beyond the Hill Sphere of the planet?

texts:
Let's imagine a moon orbiting a planet at 75000km distance (assume circular orbit). Now let's say that the Hill Sphere of this planet only extends to 78,000km (small planet in the habitable zone of a small star).

As with the moon/Earth situation, this moon is slowly drifting farther away from the planet. (ie Deimos and Mars).

What factors effect the time frame of this moon being lost so that it would be in orbit of the star and not the planet.

Does anybody know of a computer simulation showing this happen? Wondering what the trajectory and duration of the transition period would look like.


texts: I've seen some stars tonight. What could I possibly have been looking at with these pictures?

texts:
The weather tonight here is great and I decided to shoot some hand-held (too bad I don't have any tripod) night mode pictures shot with P10. I have no experience in stars observation therefore I can't tell what was I looking at. But I'd definitely love to know what are those. Forgive me if the quality of pictures are bad; I've tried my best. At some point I found some of the stars look clear on my phone but fainter over my laptop monitor.

Picture 1 - Taken under hand-held night mode: ISO/400 S/0.5s F/2.2

Picture 2 - Taken under hand-held night mode: ISO/800 S/0.5s F/2.2

Latitude, Longitude & Compass details: Right is for P1; left belongs to P2. Keep in mind that it may not 100% accurate.

Time: 22 Dec 2018 / 22:52 P.M. UTC+8
Location: Ampang, Malaysia.

Star labelled 4 is the only brightest among the others. It shines and flickers so bright that visible very well with naked eyes. The rest are barely to be seen if not focus well.


texts: impossible to download some packages on python

texts: i am new to python i will like to install some packages like einsteinpy, astropy, and several others but the error message is always the same, i think i have a problem with the pip version but i dont don't know how I can fix this. here on the attached photo,


texts: Why are some planets so close to their star?

texts:
"Kepler-10b orbits its parent star once every 0.84 days, which means
  it is more than 20 times closer to its star than Mercury is to our
  sun" space.com

Are there any accepted theories that successfully predicted that so many planets would be found so close to their star?

Edit: This is especially a question, since it seems that planets are moving away from their star. newscientist.com


texts: At what distance could a supernova damage the Earth's ozone layer?

texts: As from my latter question it seems Betelgeuse might be much closer than the usually presumed 640 light years. It might be as close as ~440 ly. Suppose it is, would this have any dangerous effects on the Earth's ozone layer (damaging it dangerously) and on orbital spaceflight? How close would a star like Betelgeuse have to be to have dangerous effects on the ozone layer and above (and thus on humans and other life on Earth)?


texts: Radial Velocity Graph and Rossiter Mclaughlin Effect

texts: I am trying to understand the effect of the Rossiter–McLaughlin effect on radial velocity plots. Can someone help me and explain what exactly is happening in the red circle?


texts: If you are standing on the shore and it is high tide, what are the two possible locations for the moon relative to where you are?

texts: Science Class If you are standing on the shore and it is high tide,


texts: Why do the Pleiades look clearer when viewed indirectly?

texts:
I've noticed that the Pleiades look bright in the sky, but when I look directly at the constellation, it suddenly goes dim and it's more difficult to make out the individual stars.

Why does this happen, and is there a way to "beat the system" by both being able to look directly at a star and see it clearly.


texts: Is there a database for identifying the falling light I saw?

texts:
I witnessed a light fall from the sky. It was visibly about as bright as Mars, so brighter than meteorites I've seen, was just a point of light with no tail, and traveled much much faster than an aircraft. The horizon was blocked by a house, so I do not have any frame of reference for how far away it hit the ground (it could as well have been 3 miles or 10,000 miles away), but I do know the coordinates for where I was, and the direction is was relative to my position.

Is there some database I can use to search space events to learn what it was that I saw?


texts: Why do these astronomers say that "Carbon chains with more than nine atoms are unstable"?

texts:
The Phys.org article Interstellar iron isn't missing, it's just hiding in plain sight links to the paywalled article On the Structure, Magnetic Properties, and Infrared Spectra of Iron Pseudocarbynes in the Interstellar Medium (Tarakeshwar, Buseck and Timmes, Arizona State University), and says:

The new work may also solve another longstanding puzzle. Carbon chains with more than nine atoms are unstable, the team explains. Yet observations have detected more complex carbon molecules in interstellar space. How nature builds these complex carbon molecules from simpler carbon molecules has been a mystery for many years.
Buseck explained, "Longer carbon chains are stablized by the addition of iron clusters." This opens a new pathway for building more complex molecules in space, such as polyaromatic hydrocarbons, of which naphthalene is a familiar example, being the main ingredient in mothballs.
Said Timmes, "Our work provides new insights into bridging the yawning gap between molecules containing nine or fewer carbon atoms and complex molecules such as C60 buckminsterfullerene, better known as 'buckyballs.'"

Question: On Earth we don't see a big difference between hydrocarbon chains with lengths below and above 9 (think kerosene, wax...), why is there such a cutoff in stability in interstellar space? What is it about hydrocarbon chains longer than 9 atoms that makes them unstable there but not here?


texts: I am struggling to calculate the redshift error of a galaxy. I was able to determine the redshifts of the absorption and emission lines from the galaxy's spectrum and then took the average of the redshift of the lines to determine the (overall) redshift of the galaxy. I know "error" is associated with the true and measured values but I don't know the true value of the redshift of the galaxy. Is there an explicit formula or something that I can use to calculate the redshift error of the galaxy without reference to the true redshift of the galaxy? All I know are the emitted and observed wavelengths of the emission and absorption lines of the galaxy spectrum.


texts: Strange asymptotic behavior on the Kozai-Lidov cycles

texts:
So, in Naoz et al. (2013) is described the Kozai mechanism on three-body systems. I've tried to solve the equations (B1) - (B17) for the example described in the Figure 3 and I got something like this:

Note that I got the exact same result until $\sim$ 33 Myr (black arrow), then the cycles change radically. Is this usual?. Do Kodai cycles tend to change over time like this or they actually remain with the same "shape" as time goes by? If the latter, isn't a way to predict the behavior of Kodai cycles as t tends to infinity?


texts: Could photons orbit a black hole if fired at the correct angle?

texts: My experience with Kerbal Space Program gives me the impression that it's impossible to transfer from a stable orbit of one body to the stable orbit of another without deceleration. But is this true for a photon? Could light enter a stable orbit of a black hole if fired at the correct angle from a distance?


texts: Possible occultation of HIP 99314 and Ganymede

texts:
We all are of course looking forward to see the great conjunction of Jupiter and Saturn. So I looked up the timeline of close approach in Stellarium and I saw that there might be an occultation with Ganymede and HIP 99314 (JD = 2459204.532313, date = 2020-12-21, 00:46:31 UTC). The star has a magnitude of around 7.5 and Ganymede has a magnitude of around 5.47 (combined magnitude is 5.31 -> change in magnitude is 0.16), so it might be interesting to see the perceivable dim (maybe not with the naked eye, but maybe on camera). Unfortunately, I don't think that Stellarium is built with that precision, so I am asking your opinion about this situation.
There are 2 options:

Stellarium is built with that precision and this event is even more rare than previously thought.
Stellarium is wrong: there are other data that don't support its results.

Help is very appreciated, but let's hope for the best.
Thank you!


texts: Multiple star-like looking objects moving across the sky

texts:
I know this question has been asked before, and not only on this resource. But none of the answers look satisfactory to my case.

So yesterday shortly after dusk, rouhgly around 18 UTC (local 21PM, I live in Ukraine), I observed a number of objects moving across the sky. In total, I counted 12 of them.
They were moving at more or less the same speed, though some of them were a bit faster (distance shortened), distances between objects vary, but in general the distance was rather big - during the movement, the first one disappeared while the last one had not yet arrived. I counted around 7-8 of them visible at the same time at the beginning, stretched across the sky. The latter ones were a bit delayed. It took several minutes for one object to cross the sky, like 3-4. Sadly, I wasn't smart enough (or too fascinated) to take exact measurements.

Their brightness reminded very much brightness of clearly visible stars with naked eye. I woud say somewhere around 1m (though this is very inaccurate), or maybe the brightness of Betelgeuse (but without reddish). (not as brigh as Venus, less bright then Sirius which was clearly visible at the time). The objects did not leave any trails and did not blink.
Jokes aside, I did not consume any drugs, alcohol or medicine that could change my perception of reality. Moreover, my significant one was nearby and also observed it.
So what could it be?

Not a flock of birds - they do not fly stretched like that, and are not so bright at night.
A group of satellites? Then, why are they so stretched? Is there a group that consists of 12 (or there might be more) units? What's the reason for some of them to be faster?
Airplanes. These ones did not blink, and in general moved faster then airplane moves across the sky (unless it was some hypersonic aircraft group that do not have any blinking/colored lights on them).
Some other flying objects.

Thanks!


texts: How is an exoplanet characterised as "Earth-Like"?

texts: Which features or characteristics are taken into account to characterize an exoplanet as "Earth-Like"?


texts:
Section 4.1.2. CHEOPS in Six transiting planets and a chain of Laplace resonances in TOI-178 says

Due to the low-Earth orbit of CHEOPS, the spacecraft-target line of sight was interrupted by Earth occultations and passages through the South Atlantic Anomaly (SAA), where no data were downlinked. This resulted in gaps in the photometry on CHEOPS orbit timescales (around 100 min). For our observations of TOI-178, this resulted in light curve efficiencies of 51%, 54%, 65%, and 86%. For all four visits, we used an exposure time of 60 s.

CHEOPS is in a ~700 km dawn-dusk sun-synchronous 92.8° inclination orbit aligned with local sunset/sunrise (06:00, 18:00) and an image of the South Atlantic Anomaly at 560 km in the 1990's is shown below. It is also discussed in Space SE's

What is the South Atlantic Anomaly, and what can I do to protect satellites from it?
Why would Space Cube 1.0 have so many On-Orbit Upsets over South America?

We can probably assume that the enhanced energetic charged particle flux due to Earth's lower magnetic field in that region of space has something to do with it, but what exactly?
Questions:

Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry? Is this out of an abundance of caution at each investigator's discretion, or is it standard, or at least recommended to do so?
Is the problem enhanced noise in the focal plane due to charged particle tracks in the CCD itself, or the potential for digital disruptions like flipped bits in registers or solid state memories? In other words, is the problem mostly analog or digital?

note: CHEOPS observations are already constrained by needing to shield its cryogenic focal plane from radiative heating from both the Sun and the Earth in low Earth orbit:

For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS?
Will CHEOPS use DSN or ESA ground stations, or both?
How many flavors do dawn-dusk Sun-synchronous orbits come in?
Wikipedia's CHEOPS Instrument System (CIS):

he required photometric precision will be achieved using a single frame-transfer, back-illuminated CCD detector from Teledyne e2v with 1024 × 1024 pixels and a pixel pitch of 13 µm. The CCD is mounted in the focal plane of the telescope, and will be passively cooled to 233 K (−40 °C), with a thermal stability of 10 mK.

South Atlantic Anomaly at 560 km in the 1990's from source


texts: 'Very High Noon' - Sun directly overhead

texts:
Twice a year the sun is at a certain angle south (or north) of the equator between The Tropic of Cancer and The Tropic of Capricorn.
At that time the sun will be exactly overhead for people at the latitude (and longitude).
Is there a table or - better - a model that takes
Latitude (and Year) as input
and responds with
Date, Time, and Longitude as output.?
-The 'closest' 'midday' (solar time) (before or after) will be the closest people at that latitude - round the world - will come to experience the sun 'directly' overhead.
You could call it 'Very High Noon'.

vj


texts:
I would like to start from the very basic such as Pythagoras Theorem.
If I want to calculate distance between two points in a right-angled Triangle We use
$c^{2}= a^{2}+b^{2}$; Using this We are quite capable to calculate the distance between two points in a 2D space. For 3D  this formula is slightly modified and We use $ds^{2}=dx^{2}+dy^{2}+dz^{2}$. But for a 4D we used to calculate distance (rather interval between two events) through this equation
$\boxed{ds^{2}=dx^{2}+dy^{2}+dz^{2}-c^{2}dt^{2}}$
Can anybody explain this modification?(looking for mathematical motivation)


texts:
I just read the answers to the question "Why are all space observatories in Chile?".  In addition to the reasons provided, one of the answers also mentions that

  ...if the telescope should be manned by humans permanently, it cannot
  be located to high due to the difficulty for humans to function at
  extremely high altitudes, ...

Which leads to my question:

  Are there any unmanned large telescopes located on Earth?

I realize that, even if large telescopes are unmanned, they probably need to be serviced by humans on a regular basis. But are humans needed permanently on site?

I also realize that, even though there are humans monitoring the measurements from space telescopes, space telescopes are unmanned in the sense that there are no people at the space telescopes manning them.


texts: Could an asteroid or a rogue planet have enough magnetism and/or gravity to spin the Earth near the speed in which they are passing by Earth?


texts: Invisible Mass Affecting the Expansion of the Universe

texts: Are there enough neutrinos & other invisible particles in a cubic light year of space to significantly affect the expansion of the universe,& at a very rough estimate without taking into account WIMPS & other exotic particles for which there is no firm evidence,what would be the total mass of this quantity of matter? Only a tiny proportion of all the neutrinos emitted since the Big Bang have been absorbed,the rest are still flying around in search of a home.


texts:
Maybe a slightly weird question, and maybe more appropriate for some future geophysics SE, but I feel like this is what questions like Do moons have moons? are really driving at, and it just almost gets answered there.

How massive must a rocky body be to liquify adequately to eliminate internal mass concentrations that could disrupt natural satellite orbits?

I understand geothermal energy to be a function of both gravitation and fissionable material in the planetary body, so an ideal answer would at least state its assumptions in regards to the amount of energy from fissionable material present.


texts: What is the probability of having a solar eclipse on a planet?

texts:
If we go by our planets 5/8 of our planets have solar eclipses.
What is the probability of a planet having a solar eclipse based off other simulations?
Is there a more accurate way to calculate the probability of a solar eclipse than taking the averages of our current planets statistics?
(4/79+7/62+12/27+6/6+0)/5

32.15%


texts: Is thermal disintegration of meteorites variable with lattitude?

texts:
If meteorites and bolides are aligned to the solar system,  then do they have twice as much atmosphere to go through at high latitudes?

Wouldn't we expect a higher percentage of metal meteorites further from the equator,  because they disintegrate slower in prolonged atmospheric traversal?

Image from:
https://academic.oup.com/mnras/article/359/2/551/987238

http://www.skyandtelescope.com/observing/random-meteors-arent-random02112015/


texts: Why do astrologers use the “wrong” sign?

texts:
I realize this may be the wrong site for this question... I apologize if you consider it inappropriate but hope this community knows the answer.
We just experienced the spectacular conjunction of Jupiter and Saturn (yay!). When I look at the sky, I see these two planets on the edge of Capricorn, almost in Sagittarius. Yet astrologers claim the conjunction is “in Aquarius”.

What is different about their (astrology’s) coordinate system? Any (historical) perspective would be appreciated!
PS I have no interest in astrology as a tool for predicting the future - just trying to understand where this discrepancy comes from.


texts: Is the universe simply a mass of atoms in 'void'; what does it look like?

texts: I've always been stumped with the question of what lies beyond the universe, and I've come to the firm conclusion - void (i.e. completely empty).  Does it make sense then to think of the universe as a finite 'blob' of atoms (the completely full) flying around each other in nothing?  Furthermore, and more importantly, does it have a shape like a sphere?


texts: Why don't we have 2 Summers and 2 Winters?

texts:
Due to Earth's elliptical orbit, its distance from Sun varies by almost 5 million Kilometers (147 million Kilometers at closest point & 152 million Kilometers at farthest point, i.e. almost 3% of the average distance).

As evident from the fact that that Venus has hotter environment than Mars due to their respective distances from the sun.

Why then Earth does not observe two winters (at farthest points) and two summers (at closest points)?

Additional Note:
I know that Earth's seasonal climate change is caused by its 23 degrees tilt that causes the sunlight density variations for the hemispheres.

But to me this 5 million Km distance seems more relevant than the 23 degrees tilt.


texts: Does Flat Universe mean even distribution?

texts:
I've read through all the other related answers to "flatness" questions but I need a bit more clarification.

I understand that a triangle in a 2D universe would not equal 180 degrees in both a closed and open universe. If I am correct, not being flat during the initial conditions would mean exponential expansion or collapse as time went by.

Given the above, could we explain flatness as uniform density of the universe, meaning all of the universe is the same approximate "consistency" on a large scale (homogeneity).


texts: Can we resolve the centers of Milky way's satellite galaxies now?

texts:
Two dwarf satellite galaxies of our galaxy have large $\frac{\text{diameter}}{\text{distance}}$ values. And one of them, Canis Major Dwarf, is 8kpc away which is
the same with our distance to the galactic center.

Can we limit the masses(even they are not heavy) of their central black holes now? Like Gillessen et al. 2009, maybe we can find stars orbiting their central black holes.


texts:
I purchased a National Geographic 76/700 EQ telescope and there was a piece missing. I'm wondering if it's needed or if I should return it, if you could also tell me what it does that'd be great.

Here's an image of what is missing :


texts: How do we know Milky Way is a 'barred' spiral galaxy?

texts:
In reference to the question, "How can we tell that the milky way is a spiral galaxy?"

The answers there clearly sum up the question asked. But Milky Way is not just a spiral galaxy. It is further classified as a barred spiral galaxy.

Question: Which particular feature in the distribution of stars, or in general a feature in observations led us to believe that it is a barred galaxy?

Note: The edge-on picture is not sufficient to establish it, since a dense distribution at smaller radii could also arise from a non-uniform density on the spiral disk if modeled to fit the observations. We don't have any data from other angles or orientations.


texts:
Based on Wikipedia, we can estimate the age of a planet's surface.

How to tell a particular crater is newly formed or not? It may have sharp edge.

Different astronomical objects may have different kinds of environments which may make this more complicated.

Are there any peer-reviewed papers?


texts: Relation between core mass and red giant mass?

texts: Yes, the title might be confusing (I'd appreciate some more clarification on it :D ). I have an estimated mass of hydrogen in the core of a star, and how much hydrogen will be in the core when the star leaves the main sequence. Is there a relationship between these or a defining or approximate equation, and are there any known values for given overall mass of a star?


texts: How would rainbows appear on other planets?

texts:
Are other planets capable of producing rainbows? How would those rainbows appear? Can rain, clouds or ice from elements other than water produce rainbows?

Related: https://space.stackexchange.com/questions/34357/rainbow-space-probe


texts: Any cheap (<100 euro) tools for observing the lines in solar spectrum?

texts:
I am now reading the book ''The story of Helium and the Birth of Astrophysics''.

I really want to observe the lines (dark or bright) in the solar spectrum.

Any easily available tools for an amateur?


texts: What are Stokes parameters for galaxy shapes, how do they compare to Stokes parameters in optics?

texts:
I remember Stokes parameters as a way to completely define the polarization state of a well-defined beam of light. However several questions here refer to Stokes parameters used in characterizing the morphology of galaxies.

In Optics it's $S_0, S_1, S_2, S_3$ and in Astronomy it's $Q, U$.

I'm guessing that these are not exactly the same set of parameters. but there might (or might not) be some connection either historically or in the way some observations were made.

How did they both end up being called Stokes parameters? Is there a relationship?
How are they calculated (equation and/or short explanation please, not just a link)

Related questions:

What methods exist to calculate the ellipticity of galaxies
How to determine the ellipticity of galaxies in SDSS
What is the meaning of ellipticity of a galaxy being more than 1?
How to get ellipticities of galaxies from SDSS using casjobs?


texts: Why do we not find larger quantities of helium in the Sun's atmosphere?

texts: The Sun currently shines because H is being burned to He, and it has been doing this for about 4.57 Ga. But when we analyze the Sun's atmosphere, we do not find unusually large amounts of He. Why not?


texts:
I am trying to imagine how dark outer space is. Looking at images of cosmonauts in space proved not very useful, as the exposure makes space look completely dark.

Thinking about it, I would imagine an object would be quite well illuminated, being bombarded by provided it's not in the shadow of a planet or some other celestial body. Is that correct? How about light scattering? Is cosmic dust abundant enough for the purpose?

EDIT: @zephyr made me realize that the quantity of light is a function of position, so let's say the question is with regards to the Milky Way. Still, I would appreciate some comments on what's the situation outside the Milky Way too.


texts: As transits of Venus come in pairs, each separated by 8 years, wouldn't that imply an accurate ratio between Earth's and Venus' revolution period? I've found the ratio to be about 0.681. I presume one could see a resonance of 2/3 in that number, but as other resonances in our solar system as for example Thethys and Enceladus (4:3) are far more accurate, could it be "just" coincidental? And if so, why the regularity in the transits?


texts:
According to this explanation at the Georgia State University website,

  Axis Tilt is Critical for Life

  . . .

  The Earth's spin axis is tilted 23.5° with respect to the ecliptic, giving moderate seasons and preventing temperature extremes anywhere on the planet. . . . Not only is the Earth's angle of tilt close to the optimum value, it also seems to have been essentially constant. That is crucial for the development of advanced life.

Is this claim correct?

Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme?
Why are "moderate" seasons required for life to exist?
If humans can live at the equator on Earth where there is the least amount of tilt, why would an exoplanet with less tilt or no tilt be necessarily non-inhabitable?
Even if humans could not live on a planet without axial tilt, are there no other forms of known "advanced" life that can? We know that extremophiles exist, such as tardigrades' ability to survive in the vacuum of space.  What is the most "advanced life" that could live on a planet without axial tilt?


texts:
So I've heard that all black holes that have been observed rotate to some degree. But if it has zero radius, how is it even possible for the black hole itself to be spinning? Or is it just all the junk inside the event horizon that's rotating?
Maybe it spins with infinite angular velocity and a finite angular momentum? Please explain how this works.


texts: How many sun-like stars are there in the universe?

texts: After yesterday's announcement of the Kepler telescope finding a huge amount of newly observed exoplanets, i saw a headline claiming that as much as 22% of sun like stars in the universe have planets in their habitable zone. There are loads of stars in the universe, so the number of planets in the habitable zone has to be enourmous. But how many of the stars in the universe are about the same size as our sun?


texts: If I look beyond (about same region, higher redshift) a galaxy, will I see its progenitors?

texts:
Is there a range of redhsift dz, such that if I look at z+dz from a massive object at redshift z, I will see its progenitors?
(in a LCDM scenario of halo assembling, with smaller halos evolving towards larger ones across the cosmic time)

I am looking for a detailed explanation of this (maybe naif) question.


texts:
Whenever I see the term dust for the particles of a nebula, I ask myself whether this is actually based on some reliable measure of particle size. Of course we can all agree it looks like dust from this distance. And we all know how such appearances often lead to assumptions which are incorrect and whose incorrectness we are often blind to.

Is there any reliable evidence by which this "dust" is known not to be pebbles, rocks, boulders etc.? Which in reality would seem vastly more likely.

How might we measure size from such distances?


texts:
I've enoucntered many acronyms related between them, like AllWISE, WISE, NEOWISE, CatWISE, WISEA, WISEAR, WISEAF, WISEU, WISEP, WISEPA, WISEPC, WISEF, WISEPF, WISER, WISEWF, WISET, WISETF, WISENF... but I would like to understand the differences and what they refer to.
Here is my attempt (I would appreciate any corrections).

WISE Is the acronym for the "Wide-field Infrared Survey Explorer" that compiled the WISE catalog of 2012 which is also called the "WISE Final Release Source Catalog" (yes, the same name for the spacecraft, mission and the catalog).

AllWISE Is the name of a later data release of the WISE mission constituting the WISE catalog for the year 2013, with many improvements over the previous, extending the work done by the WISE mission even further.

NEOWISE Is the name of the extended WISE mission (using the same WISE spacecraft) approved by NASA in 2010. It also appears in the names of many asteroids since this part of the mission focused on Near-Earth Objects instead of Brown Dwarfs and cool stars and resulted in a bunch of different catalogs.

CatWISE It is a new catalog, that was released in 2019, joining data from WISE and NEOWISE parts of the WISE mission with additional dim sources.

WISEF "refers to sources that are contained in the WISE Final Release Single-exposure Source Working Database"

WISER "refers to sources that are contained in the WISE All-Sky Release Reject Table"

WISEP "refers to sources that are contained in the WISE Preliminary Release Source Catalog"

WISEPF "refers to sources that are contained in the WISE Preliminary Release Single-exposure Source Working Database"

WSIEPA "refers to sources that are contained in the Atlas Tile Source Working Database"

WISEPC "refers to sources that are contained in the Coadd Source Working Database"

WISEWF "refers to the sources that are contained in the NEOWISE Post-Cryo Preliminary Release Single-Exposure Source Working Database"

WISEAF "refers to the sources that are contained in the All-Sky Release Single-Exposure Source Working Database"

WISET "refers to the sources that are contained in the 3-Band Cryo Source Working Database"

WISETF "refers to the sources that are contained in the 3-Band Cryo Single-Exposure Source Working Database"

WISENF "refers to the sources that are contained in the 2013 NEOWISE Post-Cryo Release Single-Exposure Source Working Database"

But what about WISEA, WISEU, WISEAR and others? And why the IDs of the objects change from catalog to catalog!? I'm so confused right now.


texts: Metallicity of Celestial Objects: Why "Metal = Non-metal"?

texts:
Metallicity of objects refers to the amount of chemical elements present in it other than Hydrogen and Helium.

  Note: The other elements may or may not be actual metals in the true sense of their defintion.

But why did astronomers use such a term as metallicity? What is the history and cause behind coining such a term that may be (or rather actually is being) confused with metal-content of that celestial object ?

Is there any scientific objective or explanation to this? I do not believe it is just a random acceptance, but perhaps there is a not-so-solid motive behind doing this. If there is, what is it?


texts: What does it mean that the Sun`s rotation axis is: RA 286.13° , Dekl: 63.87°?

texts:
first post here.

I found 1 related Question, which has not solved my Problem.

SHORT QUESTIONs VERSION:

Suns axis is 7,25° tilted from the ecliptic. But in which direction?

LONG QUESTION:

Nasa defines the Suns Rotation axis as described in the title:
https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html

Why is it described with (Ra, Declination)? Is this a (Earth) Aequatorial System placed in the middle of the Sun?

If true, then this should work (What is the angle between the equator of the sun, and the plane of the Earth's orbit). As the best anwer says, the angle between ecliptical pole and sun Rotation axis should be 7.25°. I get 4°, what is going on here? Have I messed up the angle calculation?

Thanks a lot, this is making my Brain burn : D


texts: What is the temperature of the black hole?

texts: Are black holes hot or cold? I was thinking because I wanted to know if there any human sustainable temperature so we can travel in it.


texts: How long does it take for an asteroid to get to Earth?

texts: How long does it take an asteroid to get from the asteroid belt, to Earth? I know the asteroids don't go directly from the asteroid belt to Earth, but I know it's not quick, either. The time probably varies, but what's the average time?


texts:
While reading publications in astrophysics I've encountered this kind of notation (fictitious example):
$$M = 6.1^{+0.6}_{-0.1} \times 10^{30} \textrm{ kg}.$$

Several examples are here, p. 2.

I assume that the superscript and subscript refer to the right and left uncertainty of the main magnitude. I have three questions:

Can someone explain to me what these three numbers (main magnitude, superscript, subscript) refer to, in terms of the probability distribution for the true value? For example, are they the mean and 10% and 90% quantiles? Or maybe the median and 25% and 75% quantiles? Or have I misunderstood them completely?
Can someone provide early (or the first) examples of this particular superscript-subscript notation? I only knew the "$x \pm \triangle x$" notation.
Can someone provide a written reference – textbook, manual, article, or similar – where this notation is explained?

I checked this question, but it didn't really help me. I also checked the Guide to the Expression of Uncertainty in Measurement, published by the Joint Committee for Guides in Metrology (JCGM), formed by various organizations (BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, OIML). It is a standard for nomenclature and practice in reporting uncertainty, and makes important distinctions, for example between "coverage intervals" and "confidence intervals" (see JCGM 101:2008 definitions 3.12–3.16). But I can't find anything about the superscript-subscript notation there.

Cheers!


texts: Extragalactic Magnetic Fields - General Information That I'm Missing

texts:
I have started studying Extragalactic Magnetic Fields, and mainly focusing on Biermann Battery Theory. I have gathered much information on other Dynamo Theories, as well as about our current Observational and Theoretical status on Magnetic Field Genesis.

What I'm looking for though, is a general "picture" on Extragalactic Magnetic Fields, as a whole. Faraday Rotation, AGN Jets, are basically the topics I have tracked, but I'm pretty sure there are a lot of others.

So I'm asking, if someone has studied in-depth extragalactic structures and magnetic field, possibly could guide me on some of the main and hot topics around them. This way I would have more knowledge on the topic which would be great for me!

I mean just Topic-Titles, would be great!


texts: I am about to work with Ptolemy's Model for the motion of an upper Planet (Mars). I use a epicycle rolling on the deferent. As a first step, I am just interested in the shape of the trajectory, which is why I can ignore the equant, because the uniform motion arount the equant does not change the fact, that the epicycle rolls on the deferent, which is why $\phi_0^*$ can be expressed in terms of $\phi^*$, $r_A$ and $r_a$. I want to express the angle $\phi^*$ or $\phi_0^*$ in terms of $\gamma$. I assume $r_A$, $r_a$ and $c$ (distance from Earth A to the center of the deferent $B$ and distance from this center to the equant as well). How can I express $\phi^*$ geometrically?


texts: What spectral feature was used to confirm that S0-2 is "single and available"? What is Brγ?

texts:
The Phys.org article Astronomers discover S0-2 star is single and ready for big Einstein test describes the results of a careful analysis of radial velocity measurements of the star S0-2, which orbits the supermassive black hole at the center of our (Milky Way) galaxy with a period of only 16 years. The paper itself (Devin S. Chu et al, 2018) is available for open access at ArXiv, and also ApJ.

The only mention of the word wavelength is in Section 2.2:

  The radial velocity of S0-2 is measured from both data sets by fitting a Gaussian to the Brγ absorption line.

What exactly is a Brγ ("Br" + gamma) absorption line?

below: "The orbit of S0-2 (light blue) located near the Milky Way's supermassive black hole will be used to test Einstein's Theory of General Relativity and generate potentially new gravitational models. Credit: W. M. Keck Observatory (Click for full size)


texts: How long will star formation endure before the eventual heat death of the observable universe?

texts: Lord Kelvin's prediction of heat death indicates the eventual end to star formation in the observable universe. Have any astrophysicists made any predictions for the endurance of star formation in the observable universe?


texts: Many images of the Andromeda galaxy depict a yellow-red-ish bright center and then those brown "dust layers" around the center. As far as I know the bright light comes from stars, but what is this "brown dust"?


texts: Is an "observation campaign" well recognized thing in Astronomy? Is it self-declared? Are there standard ways to announce or coordinate?

texts:
The Astronomy meta question Should we have an observation-campaign tag and what is good usage advice for it? lists about 13 example questions where the tag might apply. There are several answers that evoke the term here as well, but tags go on questions (primarily).
In it I wrote:

My understanding is that it is anything between a rally for amateur observers to try to photograph something to a highly coordinated and preplanned observing plan for ground and space observatories and facilities, but that's just my impression.

A "star party" is not an observational campaign because the primary goals are outreach, fun, and general Astronomy education. A campaign would be a targeted and to some extent coordinated effort, at least to the extent that it was announced and described ahead of time. It could certainly involve amateur astronomers or even the public, but it would still presumably have at least some research or scientific goal,
Question(s):

Is an "observation campaign" well recognized thing in Astronomy?
Is it self-declared? Do people just come up with them and announce them?
Are there standard ways to announce or coordinate them?


texts: Why do we not send diggers to Mars if we think there's underground water?

texts:
A friend of mine asked me this question:

  If we have a guess that there may be underground water on Mars, why do
  we not send rovers with diggers?

I do have a few ideas, but I want alternative views by the community.